Yoshihiko Kanzawa

Highly reliable TaOx ReRAM and direct evidence of redox reaction mechanism

Highly reliable TaOx ReRAM has been successfully demonstrated. The memory cell shows stable pulse switching with endurance over 109 cycles, sufficient retention exceeding 10 years at 85degC. TaOx exhibits stable high and low resistance states based on the redox reaction mechanism, confirmed by HX-PES directly for the first time. An 8 kbit 1T1R memory array with a good operating window has been fabricated using the standard 0.18 mum CMOS process.

Demonstration of high-density ReRAM ensuring 10-year retention at 85°C based on a newly developed reliability model

A new oxygen diffusion reliability model for a high-density bipolar ReRAM is developed based on hopping conduction in filaments, which allows statistical predication of activation energy. The filament in the active cells is confirmed by EBAC and TEM directly for the first time. With optimized filament size, a 256-kbit ReRAM with long-term retention exceeding 10 years at 85°C is successfully demonstrated.

Electroforming and resistance-switching mechanism in a magnetite thin film

The electroforming and the resistance-switching behaviors in magnetite, Fe3O4, by the application of an appropriate electric field are demonstrated on a lateral device with multiple electrodes. By means of this device, both the location and the nature of the change in Fe3O4 are specified from the electrical measurements and Raman spectroscopy. The switching phenomenon is caused in maghemite, γ-Fe2O3, which is formed by oxidation of Fe3O4, near an interface of an anode. The authors argue that the switching motion is originated in a redox reaction between the Fe3O4 and γ-Fe2O3.

Fast switching and long retention Fe-O ReRAM and its switching mechanism

A novel iron oxide (Fe-O) ReRAM is proposed and its high-speed resistance-switching of 10 ns is demonstrated. The switching mechanism is confirmed as a redox reaction between Fe₃O₄ and y-Fe₂O₃. Based on this model, we have achieved long-retention characteristics by introducing Zn atoms to suppress the reduction process.

Dependence of substitutional C incorporation on Ge content for Si1−x−yGexCy crystals grown by ultrahigh vacuum chemical vapor deposition

Si1−x−yGexCy crystals were grown by ultrahigh vacuum chemical vapor deposition (UHV-CVD) using Si2H6, GeH4, and SiH3CH3 as source gases. Although the total C content in the grown crystals increased with increasing the partial pressure of SiH3CH3 gas, the substitutional C content saturated at a certain value. The maximum substitutional C content was found to change depending on the Ge content. As the Ge content was increased from 13 to 35 at. %, the maximum substitutional C content linearly decreased from 2.0 to 0.8 at. %. These results clearly demonstrate that the existence of Ge atoms prevents the substitutional incorporation of C atoms in Si1−x−yGexCy growth by CVD.

Retention Model for High-Density ReRAM

A retention model for both the high resistance state and low resistance state of the bipolar ReRAM is developed. Degradation of resistance is caused by the oxygen vacancy profile in filament changing due to oxygen diffusion.

40mΩ / 1700V DioMOS (Diode in SiC MOSFET) for High Power Switching Applications

Device technologies of SiC MOSFETs have nearly matured to the level of mass production and one of the remaining tasks is to serve better solutions in view of both costs and performances for practical systems. Elimination of external reverse diodes in inverter circuits is one of the solutions, by which total area of the SiC chips is greatly reduced leading to lower material cost. A DioMOS (Diode in SiC MOSFET) successfully integrates the reverse diode without any increase of the chip size from the original MOS transistor by utilizing an n-type epitaxial channel under the MOS gate for the reverse conduction path of the diode. The basic concept of the DioMOS has been proposed [1]; meanwhile, further reduction of the on-state resistance together with confirmation of high-speed switching is necessary for its application in power switching systems. In this paper, low on-state resistance (Ron) of 40mΩ and blocking voltage (BVds) of 1700V as well as improved switching performances of DioMOS are demonstrated. The measured results suggest DioMOS to be satisfactory for practical use.

Thermal-annealing dependence of C-related atomic configuration in Si1−x−yGexCy crystals grown by ultra-high-vacuum chemical-vapor deposition

Si1−x−yGexCy crystals were grown by ultra-high-vacuum chemical-vapor deposition and their thermal-annealing behaviors were studied by infrared (IR) absorption measurements. The present samples exhibited absorption peaks originating from local vibrational modes of substitutional C atoms with two different atomic configurations; one is a C atom with four Si neighbors (Si4 configuration) and the other is with one Ge and three Si neighbors (Si3Ge1 configuration). As the annealing temperature was increased from 700 to 950 °C, the absorption peak of the C atoms with Si3Ge1 configuration increased in intensity. This clearly demonstrated that thermal annealing leads to the formation of Ge–C bonds. Moreover, the composition dependence of the IR spectra suggested that Ge–C bonds are formed to reduce the strain energy of the Si1−x−yGexCy crystals.

Reduction of neutral base recombination in narrow band-gap SiGeC base heterojunction bipolar transistors

Narrow band-gap SiGeC base HBTs were fabricated by Si compatible processing. By introducing 0.3% carbon into the SiGe layers, lattice strain was decreased and neutral base recombination localized at the collector-base heterojunction was significantly reduced.

(CD-5) TDDB breakdown of th-SiO 2 on 4H-SiC: 3D SEM failure analysis

We analyzed SiO2 void volumes formed on samples which failed during TDDB tests by constructing three-dimensional (3D) images in FIB-SEM system. We succeeded in visualizing the breakdown spots within SiO2 on 4H-SiC substrates and provided clear evidence of the influence of SiO2 void volume on 4H-SiC TDDB failure (ex. Tbd or Rbd). The small SiO2 voids play a very important role in the degradation of TDDB reliability. The present 3D SEM technique applied to th-SiO2 on 4H-SiC can be very useful in the investigation of TDDB failure mechanism.